Electrical testing instrument.



No; 896,300. PATENTEDAUG, 1a, 1908.

0. DE w. JARVIS. ELECTRICAL TESTING INSTRUMENT.

APPLICATION FILED JUNE 19, 1907.

" sra'rns PATENT enrich.

CAMERON DE WI TTJARVIS, OF WINTHROP, MASSACHUSETTS, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

EiLEo'rnIoAL TESTING msraumn'r.

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Y Specification of Letters Patent.

Application and June 1c, 1907. SeriaiNo. 379,804..

To all whom it may concern:

Be it known that I, CAMERON DE W. JAR- vrs, residing at Winthrop, in the county of Suffolk and State of Massachusetts, have invented certain Improvements in Electrical Testing Instruments, of which the following is a specification.

This invention relates to electrical testing devices, and particularly to portable instruments adapted for the approximate determination of differences of potential. Itsv principal objects are to provide such a device which is so small and compact as to be suitable for carrying in the pocket, the indica tions of which may be readily observed, and which will not be liableto getoutof order.

In the accompanyingdrawing, in which same characters are applied to like parts throughout'the several views, Figure 1 is a side elevation of, one embodiment of the invention; Fig. 2 is a longitudinal section on the line 2-2 of Fig. 1; Fig. 3 is an end eleva-- tion looking fromth'e ht in Fig.1 but with p the casing-cap remove and Fig. 4 illustrates diagrammatically the circuit arrangement of the instrument. i T p The numeral '10 designates a body portion of generally cylindrical form, preferably of some such insulatingmaterial as hard rubher, which serves as a support for the other elements of the device. Near one end of the body portion is shown a circumferential integral flange 11, while the opposite extremity is reduced and threaded at 12 to receive a head 13. The portion of the body extending.

beyondthe flange may be oppositely flat 'tened at 1.414 to receive a pair of metallic spring arms 15-45, retained inplace byan;

encircling insulating ring 16 under the inner edge of which the spring may be bent.' These springs furnish a socket for indicating means, which, as illustrated, consists-of an incandescent lamp 17 having its inner end projecting into a depression in the body, portion. The leading-in wires of the lampare soldered to external side plates 1818 for cooperating with the springs, an insulating ring 19 surrounding these elements and insuring perfect contact.

That portion of the body lying between the flange 11 and head 13 provides a spool for resistance coils R by which the current flowing through the lamp and the resistance of the entire device may be regulated, and which are conductively associated with the socket springs and with connecting posts or termirials 2020 secured to the outer end of the heat. Two coils 21 and 22 are preferably employed, the relation of these being best shown in Fig. 4.- of the drawing. The coil 21 is included in conductors 2323 which unite the terminals 20 with the springs 1515, and

is of such resistance that the resistance of the device as a whole is approximately that of the apparatus through which the source of electrical energy to be tested is to supply current. The coil 22 is bridged across the conductors 23 in parallel with the lamp, and its resistance is so chosen with reference to. that of the filament thatwhen the lowest 'permissible voltage is appliedto the terminals of Patented Aug. 18, 1908.

the device, the -'l'amp will receive suflicient.

current to glow strongly enough to be plainly visible to the average observer, whil the application-v of the maximum difference of potential which the gage is to test will not result in injury to the filament.

The head 13 has an innerportion 24 ofless diameter than the remainder, and fitting over this and the flange 11 is a protecting sleeve or casing 25, conveniently of metal, which may be secured in place by screws 26 taking the head. The casing has a continu-' ation furnished by a cap 27, of hard rubber or the like, which is closed at its outer end to cover the lamp, while. a reduced portion 28 enters thesleeve and is secured thereto by screws 29. I The ca is provided with a con tracted opening, w "ch, as illustrated, is in the form of a transverse slot 30 inclined from which are spring cli s 33,- bywhich the de' vice may be attache to the-apparatus to be tested.

- This instrument is especially useful in se-,

curing an indication of the condition of the limited number of cells of battery employed in connection with telephone transmitters. The lamp and coil-resistance being properly being in circuit not glow or its light is barely seen the voltage of the cells has so fallen off that they need to be renewed. Obviously, this is but one of many possible applications of my invention, which may be effectively em loyedwhenever a highly portable and easi y read instrument of no great accuracy or range is required.

I claim as my invention'and desire to secure by Letters Patent: I

1. An electrical testing device comprising a body portion, indicating means mounte upon one end of said body portion', a resistance coil surrounding the body portion and with the indicating means, and a casing extending coil and having a separable cap inclosing the indicating means and being provided with a contracted opening.

ortioned for the desired purpose and the over the resistancev 2. The combination with an'elongated insulating support provided at one extremity with a socket adapted to receive an incandescent lamp and at the opposite extremity with connecting terminals, of a regulating coil surrounding the sup ort between the socket and terminals an being connected therewith, and a casing carried by the support and inclosing the socket and coil but aving an observation opening adjacent to.

said socket.

3. A testing device for telephone transmitter batteries, comprisin' an incandescent lamp, a resistance in paral el with the lamp whereby the amount of current through said 7 lamp is controlled, and a resistance connected in series with the lam and rendering the total resistance of the g mately that of a transmitter.

In testimony whereof I have name to this specification in the presence of two subscribing witnesses, this twelfth day of June, 1907 y I CAMEBON Witnesses GEO. WILLIS PiERcE, JOSEPH A.. GATELY.

DE WITT JARVIS.

evicew approxisigned my 

